The sudden appearance of blood clots outside of a regular menstrual cycle can be a confusing and concerning experience. While the passage of small clots during a heavy period is common, any unscheduled clotting is classified as a form of Abnormal Uterine Bleeding (AUB). AUB can range from light spotting to heavy flow, originating from hormonal changes, structural issues within the reproductive tract, or early pregnancy complications. Understanding the source of this irregular bleeding is the first step toward appropriate medical management.
Causes Related to Hormonal Imbalance and Medication
Hormonal fluctuations are a frequent non-structural cause of unscheduled uterine bleeding and clotting. The menstrual cycle is governed by the balance between estrogen and progesterone, which controls the growth and shedding of the uterine lining (endometrium). When this balance is disrupted, the lining can grow too thick and shed irregularly.
A lack of ovulation (anovulation) is a primary culprit, commonly occurring during perimenopause or in conditions like Polycystic Ovary Syndrome (PCOS). Without the regular release of an egg, the body does not produce adequate progesterone to stabilize the estrogen-driven growth of the uterine lining. This “unopposed” estrogen causes the endometrium to build up excessively until it outgrows its blood supply and sheds sporadically, resulting in heavy, clotted bleeding.
Dysfunction of the thyroid gland can also impact the regularity of the menstrual cycle. Hypothyroidism (an underactive thyroid) has been linked to heavier menstrual bleeding (menorrhagia) and may interfere with the body’s normal blood clotting factors, potentially leading to pronounced clotting outside the expected cycle.
Medication changes, particularly those involving hormonal birth control, are another common cause of unscheduled bleeding. When starting a new pill, patch, or ring, or changing to a lower-dose formulation, the body takes time to adjust to the new hormone levels. This adjustment period can lead to breakthrough bleeding, which may include clots as the body sheds the lining irregularly. High doses of hormones, such as those in emergency contraception, can also cause an abrupt shedding of the uterine lining, resulting in a heavier bleed with clots.
Structural and Anatomical Conditions
Physical changes within the uterus or cervix can interfere with the normal shedding process, causing localized bleeding and subsequent clotting. These structural issues are often evaluated using the PALM-COEIN classification system for Abnormal Uterine Bleeding, where “PALM” represents Polyps, Adenomyosis, Leiomyoma (fibroids), and Malignancy.
Uterine fibroids (leiomyomas) are non-cancerous muscle tumors that grow in or on the wall of the uterus. Submucosal fibroids, which bulge into the uterine cavity, are the most likely to cause abnormal bleeding and clotting. These growths distort the uterine cavity, increasing the surface area of the lining that sheds, and interfere with the uterus’s ability to contract and stop bleeding. This increased volume of blood loss overwhelms the body’s natural anti-clotting mechanisms, resulting in the passage of large clots.
Endometrial polyps are benign, fragile overgrowths of the uterine lining tissue attached to the uterine wall by a stalk. Because polyps are highly vascular and sensitive to hormonal fluctuations, they are prone to irritation and bleeding, often presenting as spotting or light bleeding between periods. The blood from these fragile vessels can pool and clot before being expelled.
Adenomyosis occurs when endometrial tissue grows into the muscular wall of the uterus (myometrium), causing the uterus to become enlarged. This misplaced tissue responds to the menstrual cycle, leading to chronic inflammation and thickening of the uterine wall. The result is often severe cramping and heavy bleeding, including the passage of large blood clots due to disorganized shedding and impaired muscle contraction. Other anatomical sources of irregular clotting include cervical polyps or inflammation of the cervix, which can cause bleeding upon physical contact (e.g., during intercourse or a pelvic exam).
Clotting Related to Early Pregnancy and Miscarriage
Bleeding and clotting outside of a period may indicate a pregnancy-related event, regardless of whether a pregnancy was confirmed. Any unexplained clotting should prompt a pregnancy test, as treatment protocols differ significantly for a gestational cause.
Implantation bleeding occurs when a fertilized egg embeds into the uterine lining, resulting in light spotting around the time a period is expected. This bleeding is usually light pink or brown, lasts only a day or two, and does not contain clots. The presence of true blood clots suggests a heavier bleed, which is not characteristic of simple implantation.
The passage of clots and tissue can be a sign of a threatened or complete miscarriage (the loss of a pregnancy before 20 weeks). Bleeding associated with a miscarriage is often heavier and more prolonged than a regular period, accompanied by cramping, and may involve the passage of grayish or pinkish tissue along with dark red clots. The size of the clots and recognizable tissue depends on the stage of the pregnancy.
A serious, though less common, cause is an ectopic pregnancy, where a fertilized egg implants outside the uterus, most often in a fallopian tube. This condition is a medical emergency and may present with vaginal bleeding, often accompanied by severe, sharp pain on one side of the abdomen, shoulder pain, or dizziness. The combination of abnormal clotting and severe pain requires immediate medical attention.
Recognizing Warning Signs and Medical Evaluation
While many causes of irregular clotting are benign, certain symptoms require an immediate call to a healthcare provider. Bleeding heavy enough to soak through more than one sanitary pad or tampon in an hour for several hours is a sign of acute blood loss and requires urgent attention. The passage of clots larger than a quarter (or golf-ball sized) indicates a significant issue with bleeding control.
Other warning signs include severe pelvic or abdominal pain, especially if sharp or one-sided, which can suggest an ectopic pregnancy. Signs of systemic illness, such as fever, chills, dizziness, or fainting, also prompt immediate medical evaluation, as these can indicate severe blood loss or an infection.
A medical evaluation for abnormal uterine bleeding begins with a detailed history and a physical examination, including a pelvic exam. A pregnancy test is the first step for anyone of reproductive age, followed by blood tests to check for anemia, thyroid function, and hormone levels. Imaging is often used to check for structural causes, with transvaginal ultrasound being the most appropriate initial test to visualize the uterus and ovaries. If the initial ultrasound is inconclusive, a Saline-Infusion Sonohysterography (SIS) may be performed to evaluate the uterine cavity for polyps or submucosal fibroids.